This research is investigating at what points in the metabolism of nucleic acids the two teratogens 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine (5-FUDR) interact with Strong's luxoid gene (1st) and its modifiers to disturb limb development in two strains of mice selected for low/high expression of 1st's effects on the limb skeleton. The research is also investigating the bases in nucleic acid metabolism of the potentiating effects of uracil on the teratogenic effects of 5-FU and the protective effects of thymidine on the teratogenic effects of 5-FUDR. Also being investigated is the question whether Strong's luxoid gene or its modifiers have any effect on nucleic acid metabolism. In these studies, radioactive tracers are being used.